Partial (unicompartmental) knee replacement

Overview

Front view of the knee showing the femur, tibia, patella, cruciate and collateral ligaments, and the medial and lateral menisci. — **Knee anatomy.** The knee is the meeting point of the thigh bone (femur), shin bone (tibia), and kneecap (patella). Four ligaments hold it together — the ACL and PCL inside the joint and the MCL and LCL on the sides — and two C-shaped menisci cushion the joint surfaces.

The knee has three compartments — medial, lateral, and patellofemoral. When arthritis affects only one, resurfacing the entire joint may be overkill. Partial knee replacement targets just the worn compartment and preserves the ACL, PCL, menisci, and healthy cartilage of the other two compartments. Patients often describe the result as feeling more like their own knee.

How the Procedure Works

AP and lateral knee X-rays after unicompartmental (partial) knee replacement showing implants only on the medial (inner) side of the joint. — **Post-op partial knee replacement.** Only the worn compartment is resurfaced. The kneecap and the opposite side of the joint stay intact, which is why many patients say a partial knee feels more like their own knee than a total replacement does.

We resurface only the worn compartment — typically the medial femoral condyle and its matching tibial plateau — with a smaller metal-and-polyethylene implant. Both cruciate ligaments, the opposite compartment, and the patellofemoral joint are left entirely untouched. Patient selection is the most important factor in outcomes: the ACL must be functional (it drives the knee's natural rollback kinematics that a partial replacement relies on), the deformity must be passively correctable, and the other compartments must have intact cartilage. Implant sizing and tibial slope are set intraoperatively to match the native knee — the goal is a component that participates in normal knee motion rather than constraining it. The preserved ligament proprioception is why patients often describe partial replacement as feeling more like their own knee than a total replacement does.

When to Consider Partial (unicompartmental) Knee Replacement

Partial (unicompartmental) knee replacement is generally offered when symptoms, imaging, and a trial of non-operative care together point to surgery as the next step. The typical picture includes:

Single-compartment arthritis
Cartilage loss isolated to the medial or lateral compartment, confirmed on weight-bearing X-rays.
Intact ligaments
A functional ACL and PCL — the ligaments must still stabilize the knee for a partial replacement to work.
Correctable deformity
A varus or valgus deformity that reduces passively, suggesting the other compartments are still healthy.